In those facilities and equipment which utilize a superconductor, a conductor, generally called a cryostatic stabilizer, is provided on and around the superconductor to protect the superconductor by by-passing the electric current to the conductor when the state of superconductivity returns partly or completely to the state of normal conductivity due to external thermal, electric or magnetic disturbance. The transition from the state of superconductivity to the state of normal conductivity (usually called as "QUENCHING") is accompanied by I.sup.2 R heat generation (wherein I means electric current and R means electric resistivity of the conductor) in the normal regions of the conductor where current flows.
High purity aluminum, because its electric resistivity is remarkably low at ultra low temperature and in magnetic field, has been discussed for possible use as such a cryostatic stabilizer. [F.R. Fickett, "Magneto-resistivity of Very Pure Polycrystalline Aluminum", Phy. Rev. B. Vol. 3, No. 6, 1971, p1941. "Superconducting Magnetic Energy Storage" Vol. 1: Basic R&D 1984-85, EPRI GS-7053, published by the Electric Power Research Institute in Nov. 1990.]
The use of a cryostatic stabilizer made of high purity aluminum is planned for Superconducting Magnetic Energy Storage (SMES) devices. But in such facilities which store large quantities of electric power, hoop stresses are caused by the flow of current through the magnet, and when electric charging and discharging are repeated, cyclic tensile stress and compressive stress are given repeatedly to the superconductor and the cryostatic stabilizer.
It is known that such cyclic stress which includes a plastic strain component at ultra low temperature gives an adverse influence on high purity aluminum at ultra low temperature in the form of an increase in electric resistivity. [Advances in Cryogenic Engineering. 22, 486-489 (1976).]
Therefore, for those applications in which cyclic strain is given at ultra low temperature to the cryostatic stabilizer of high purity aluminum, the high purity aluminum conductor component ought to be of a relatively larger cross section in view of a possible increase in electric resistivity of the cryostatic stabilizer when in use, or the conductor should be so designed as to reduce plastic strain of the cryostatic stabilizer under the same stress by increasing the design strength of the structural materials of SMES.
However, the above countermeasures require a large amount of materials when adopted for such large structures as utility scale SMES and are therefore very costly.
Further, it is known in the report of the International Conference on Cryogenic Materials, Applications and Properties, Shenyang, People's Republic of China, Jun. 7-10, 1988 that in the case of a high purity aluminum conductor with the same purity as that of the high purity aluminum in the present invention used at ultra low temperature, its electric resistivity under cyclic strain does not remain low enough for the stabilizer if the cyclic strain range is too high.